1. Field of the Invention
The present invention relates to a driving device provided with a stepping motor as a drive source, a light-amount adjusting device such as a diaphragm device that uses the driving device, and a camera provided with the light-amount adjusting device.
2. Related Background Art
Known light-amount adjusting devices, which are used for video cameras and the like, include: an actuator of a galvano-system that causes two diaphragm blades to perform opening/closing operation according to an amount of current applied to a drive coil; a Hall element for detecting a diaphragm position; and a circuit that outputs a control signal of the diaphragm position. However, this system has the following disadvantages: 1) it has a complicated structure due to its large number of parts such as a sensor for position detection (Hall element), a circuit therefor, and a control circuit; 2) it is necessary to lower the resistance value of the drive coil in order to add a shutter function, which leads to a reduction in power consumption for the diaphragm operation. In view of this, a diaphragm device using a stepping motor has been proposed.
For example, Japanese Patent Application Laid-Open No. H07-111617 discloses a diaphragm device that uses a stepping motor, and a reset sensor is provided at a full close position of the diaphragm. When the power source is turned OFF, blades of the diaphragm are moved further toward the close side and stopped. Accordingly, when the power source is turned ON at the next time, the blades are moved toward the open side, making it possible to positively perform reset detection in a short period of time. Further, means for judging whether the blades are at a position closer to the close side than the reset sensor, or closer to the open side is provided to determine which direction the blades should be moved when a reset operation is performed. Accordingly, the reset operation is performed positively in a short period of time.
Further, in the diaphragm device disclosed in Japanese Patent Application Laid-Open No. 2002-107792, diaphragm blades are driven by using a stepping motor, and an initial reference position is determined through mechanical abutment. With respect to rotation of the rotor, a mechanical angle is set so as to fall within one rotation of an electrical angle. Accordingly, it is possible to dispense with a reset sensor and to positively detect the blade position.
However, the diaphragm device disclosed in Japanese Patent Application Laid-Open No. 2002-107792 involves such a disadvantage that the mechanical angle is limited by the electrical angle, which increases movement of the blades per step. As a result, it is impossible to perform a minute operation of the diaphragm device. On the other hand, if the mechanical angle is made larger so as to fall within a range that is not limited by the electrical angle, a reset operation is necessary. Accordingly, in order to positively find an amount of its diaphragm without providing the reset sensor, one possible method is to bring a drive unit into abutment with one of the mechanical ends such as stoppers, and to find the amount of its diaphragm by using as the reset position the position of the mechanical ends.
However, if the reset operation is performed through mechanical abutment as described above, no means that uses an electrical signal or the like is provided for checking whether or not the reset operation is performed. Accordingly, a stroke larger than a full stroke between the closed position and the opened position is driven to bring the blades into abutment with the mechanical end to determine that the position where the blades are stopped is the reset position. With this structure, not only does an impact at the time of abutment causes an unpleasant noise, but also damages are caused to the diaphragm device by repetitive reset operations, and that may affect the function thereof.
If the blades are stopped in the vicinity of the reset position when the power source is turned OFF or the like as in the diaphragm device disclosed in Japanese Patent Application Laid-Open No. H07-111617 above, and if the reset operation is performed through mechanical abutment as in the diaphragm device disclosed in Japanese Patent Application Laid-Open No. 2002-107792, a time period increases where the blades are meaninglessly driven while being in abutment with the mechanical end when the reset operation is performed, causing the above unpleasant noise for a longer period of time, and also causing damage to the function.